The fluidized-bed method has long been used for generating heat or, in more general terms, for the treatment of granular materials in the form of particles of relatively small dimensions. Treatment is carried out inside a fluidization chamber consisting of a vertical elongate enclosure which is fed with solid materials in the form of particles and at the base of which is injected a fluidization gas which circulates upwards at sufficient speed to place the particles in suspension, the critical fluidization speed depending on the size and density of the particles.
Because of the high conversion yield, fluidized beds are used particularly for the combustion or gasification of hydrocarbon materials, such as coal, lignite, wood, etc. If, for example, coal is burnt in a boiler, the latter can consist of an enclosure of large dimensions, comprising a lower part in which combustion takes place in fluidized beds, and an upper part through which pass the hot gases produced by combustion and in which a bank of heat-exchanger tubes is located.
Other treatments can also be carried out in fluidized beds, and to increase the efficiency of the treatment, the gases can be circulated at considerable speed, for example at a speed greater than 2 m/s. In this case, a substantial proportion of particles is conveyed upwards together with the gases, and it is possible to distinguish within the fluidization chamber a dense lower zone with a high concentration of particles, above which there is a dilute zone containing a large proportion of particles conveyed together with the gases towards the upper part of the chamber, the latter opening into a discharge circuit for the gases and particles carried along. The discharge circuit then passes through a device for recovering the particles carried along, usually of a cyclone type, which comprises an upper outlet for the clean gases and a lower outlet for the recovered particles, the latter being connected to the fluidization chamber by means of a circuit for recycling the particles in the fluidized bed.
Most treatments involve exothermic reactions and justify the recovery of heat by the use of an exchanger consisting of a bank of tubes, through which a heatexchange fluid passes and which is placed in contact with the hot gases and/or the particles. In conventional fluidized-bed boilers, the rate of circulation of the fluidization gas is adjusted in such a way that only the fine ashes can be carried along with the gases. It is then possible to place the heat exchanger without danger in the dilute zone above the dense fluidized bed. In contrast, if circulating-bed operation is used and if a heat exchanger is placed inside the reaction chamber, in the dilute zone of the fluidized bed, the usually abrasive particles carried along by the gases in this zone cause rapid wear of the exchanger, which also impedes the rising circulation of the gases and the carrying along of the particles. This disadvantage can be reduced by the use of a tubular exchanger consisting, for example, of a membranous wall placed along the side wall of the enclosure, of which it can moreover form the inner face, the latter being covered on the outside with a refractory covering. In this case, the heat exchanger does not impede the circulation of the gases, but it is likewise in contact with a fairly small proportion of hot solid particles, most of which are conveyed upwards in the central part of the enclosure, and then the amount of heat recovered is fairly small.
Moreover, the combustion reaction continues in the dilute zone over a height which depends on the quality and particle size of the fuel, taking into account the fluidization rate. The heat exchanger located in this zone can consequently be subjected to excessive corrosion there.
Therefore, when circulating-bed operation is used, it is generally preferred to place the heat exchanger outside the fluidization chamber, for example in the particle recycling circuit.